Since a conductive paste containing metal powder has good conductivity, the conductive paste is widely used for components of a variety of electronic devices. The conductive paste is used for forming conductor wiring of a wiring board. As the conductive paste, for Example, a paste formed by dispersing metal powder and an inorganic binder in an organic vehicle is used. To form conductor wiring, first, the conductive paste is applied to, for Example, a ceramic or a glass substrate in a predetermined pattern using a screen printer, a dispenser, and the like. Then, the applied conductive paste is baked at a high temperature so that the organic vehicle evaporates and the metal powder sinters. This forms a continuous membrane having good conductivity.
Since components of electronic devices have been densely packed, there is a need for efficiently forming a minute wiring pattern (fine pattern). More specifically, it is required to accurately form a fine pattern having a line width and a line interval of 100 μm or less. Therefore, a conductive paste for improving the accuracy of the fine pattern has been disclosed.
Patent Document 1 proposes a conductive paste, which contains spherical conductive powder having an average particle size of 0.8 μm or less and an organic vehicle. The ratio (D50/Dmin) between the median value D50 of the particle size of the conductive powder and the minimum value Dmin of the detectable particle size of the conductive powder is in the range of 2 to 5. Since using this conductive paste reduces waviness of the surface of a conductive pattern, a fine pattern having excellent shape accuracy is obtained. According to the above publication, the ratio (V1rpm/V10rpm) between the viscosity V1rpm measured with a Brookfield rotational viscometer using a No. 4 spindle at 1 rpm and the viscosity V10rpm measured with the same instrument at 10 rpm is preferably 2 to 5. Because of this characteristic, the conductive paste does not easily sag after being applied in a predetermined pattern. Thus, the form of the coated film is reliably maintained.
Also, Patent Document 2 proposes a conductive paste, which contains spherical metal powder having a particle size of 100 μm or less and a heat curable phenol resin as a binder resin. The conductive paste includes 0.01 to 5 wt % of a polyethylene resin with respect to 100 parts by weight of the metal powder. By using such a conductive paste, a fine pattern having excellent shape accuracy is formed, and therefore a high quality and high accuracy conductive circuit is manufactured.
In general, the wiring resistance of the conductive paste is required to be reduced for use in forming wiring with the conductive paste. To reduce the wiring resistance, it is effective to form a conductor wiring with a wide line width. However, widening the line width is often difficult due to the design, and there is a strong demand for narrowing the line width. Therefore, to reduce the wiring resistance, the thickness of the coated film must be increased in addition to reducing the volume resistivity of the conductive paste. Also, to increase the conductivity of the wiring, the conductive paste is required to be sufficiently sintered. However, the shrinkage rate of the conductive paste during drying and sintering is great, and the thickness after sintering is reduced to approximately 20% of the thickness when being applied. To suppress shrinkage, the proportion of the solid content such as the metal powder and the inorganic binder in the conductive paste may be increased.
However, in the conventional conductive paste, if the proportion of the solid content in the conductive paste is increased, the amount of the organic vehicle is relatively reduced. Therefore, the viscosity of the conductive paste is increased due to evaporation of a solvent in the organic vehicle, which affects the applying property of the conductive paste. For example, when forming a fine pattern on the wiring board by screen printing, a screen printing plate having a mesh with small openings is used. In this case, the increased viscosity of the conductive paste hinders the conductive paste from passing through the screen printing plate. This causes clogging of the screen printing plate. Thus, thin spots or disconnection might be created in the wiring, making it difficult to accurately form a fine pattern of 100 μm or less. In particular, when continuously printing for a long time, thin spots or disconnection are easily formed, making it even more difficult to accurately form a fine pattern. Also, when forming a fine pattern using a dispenser, the same problems as in the case of the screen printing occur since a nozzle with a small opening is used.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-139838    Patent Document 2: Japanese Laid-Open Patent Publication No. 9-310006